1. Field of the Invention
This invention relates to solid state lamps and bulbs and in particular to efficient and reliable light emitting diode (LED) based lamps and bulbs having enhanced color rendering index (CRI) through separation of different phosphor components.
2. Description of the Related Art
Incandescent or filament-based lamps or bulbs are commonly used as light sources for both residential and commercial facilities. However, such lamps are highly inefficient light sources, with as much as 95% of the input energy lost, primarily in the form of heat or infrared energy. Compact Fluorescent Lamps are more effective than incandescent at converting the electricity into light but require the use of toxic materials such as Hg such that when the lamps are disposed of these toxic materials that can pollute the environment, including underground water supplies. One solution for improving the efficiency of lamps or bulbs is to use solid state devices such as light emitting diodes (LED or LEDs), rather than metal filaments, to produce light.
Light emitting diodes generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
In order to use an LED chip in a circuit or other like arrangement, it is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, light focusing and the like. An LED package also includes electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. In a typical LED package 10 illustrated in FIG. 1, a single LED or LED chip 12 is mounted on a reflective cup 13 by means of a solder bond or conductive epoxy. One or more wire bonds 11 connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to or integral with the reflective cup 13. The reflective cup may be filled with an encapsulant material 16 which may contain a wavelength conversion material such as a phosphor. Light emitted by the LED at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly is then encapsulated in a clear protective resin 14, which may be molded in the shape of a lens to collimate the light emitted from the LED chip 12. While the reflective cup 13 may direct light in an upward direction, optical losses may occur when the light is reflected (i.e. some light may be absorbed by the reflective cup due to the less than 100% reflectivity of practical reflector surfaces). In addition, heat retention may be an issue for a package such as the package 10 shown in FIG. 1a, since it may be difficult to extract heat through the leads 15A, 15B.
A conventional LED package 20 illustrated in FIG. 2 may be more suited for high power operations which may generate more heat. In the LED package 20, one or more LEDs 22 are mounted onto a carrier such as a printed circuit board (PCB) carrier, substrate or submount 23. A metal reflector 24 mounted on the submount 23 surrounds the LED chip(s) 22 and reflects light emitted by the LEDs 22 away from the package 20. The reflector 24 also provides mechanical protection to the LEDs 22. One or more wire bond connections 27 are made between ohmic contacts on the LED chips 22 and electrical traces 25A, 25B on the submount 23. The mounted LEDs 22 are then covered with an encapsulant 26, which may provide environmental and mechanical protection to the chips while also acting as a lens. The encapsulant 26 can also comprise one or more convention materials (e.g. phosphors) that absorb light from the LED chips and re-emit light with different wavelengths of light. The overall emission from the package 20 can be a combination of light from the LEDs 22 and the re-emitted light from the conversion material. The metal reflector 24 is typically attached to the carrier by means of a solder or epoxy bond.
LEDs, such as those found in the LED package 20 of FIG. 2 can also be coated by conversion material comprising one or more phosphors, with the phosphors absorbing at least some of the LED light. The LED can emit a different wavelength of light such that it emits a combination of light from the LED and the phosphor. The LEDs can be coated with a phosphor using many different methods, with one suitable method being described in U.S. patent applications Ser. Nos. 11/656,759 and 11/899,790, both to Chitnis et al. and both entitled “Wafer Level Phosphor Coating Method and Devices Fabricated Utilizing Method”. Alternatively, the LEDs can be coated using other methods such as electrophoretic deposition (EPD), with a suitable EPD method described in U.S. patent application Ser. No. 11/473,089 to Tarsa et al. entitled “Close Loop Electrophoretic Deposition of Semiconductor Devices”.
Lamps have also been developed utilizing solid state light sources, such as LEDs, with a conversion material that is separated from or remote to the LEDs. Such arrangements are disclosed in U.S. Pat. No. 6,350,041 to Tarsa et al., entitled “High Output Radial Dispersing Lamp Using a Solid State Light Source.” The lamps described in this patent can comprise a solid state light source that transmits light through a separator to a disperser having a phosphor. The disperser can disperse the light in a desired pattern and/or changes its color by converting at least some of the light through a phosphor. In some embodiments the separator spaces the light source a sufficient distance from the disperser such that heat from the light source will not transfer to the disperser when the light source is carrying elevated currents necessary for room illumination. Additional remote phosphor techniques are described in U.S. Pat. No. 7,614,759 to Negley et al., entitled “Lighting Device.”
The coated LEDs, LED packages and solid state lamps described above can utilize more than one type of conversion material, such as phosphors, to produce the desired overall emission temperature and CRI. Each of the phosphors can absorb light from the LED and re-emit light at a different wavelength of light. Some of these conventional arrangements can utilize a green/yellow phosphor, in combination with a red phosphor, with these phosphors typically absorbing blue LED light and emitting green/yellow and red light, respectively. The re-emitted light can combine with blue LED light to produce the desired emission characteristics.
These conventional arrangements typically mix the different phosphors together at one location, such as in the LED coating, LED package encapsulant, or lamp remote phosphor. One disadvantage of mixing the phosphors together is that there can be significant “cross-talk” or “overlap” between the emission and excitation spectrum for the different phosphors, which can negatively impact the CRI and emission efficiency for the combined emitted light. FIG. 3 shows graphs 30 showing one example of the emission and excitation characteristics for conventional phosphors that can be mixed together. The first graph 30 shows the red phosphor excitation spectrum 32, green phosphor emission spectrum 34, and the red emission spectrum 36. The second graph 40 shows the same red phosphor emission excitation spectrum 32, yellow phosphor emission spectrum 42, and the same red phosphor emission spectrum 36. The shaded overlap regions 38, 44 show the portion of the green and yellow emission spectrums 34, 42 that overlap the red excitation spectrum 32. This overlap can result in “re-absorption” of the converted yellow/green phosphor light by the red phosphor. This converts a portion of the yellow/green that would otherwise contribute to overall emission, to red. In lighting components using these phosphors to produce a white light combination from the LED and phosphors, the re-absorption distorts the resulting white light on the black body curve of a CIE graph such that the yellow/green peak emission can shift to red, and the red peak can shift to blue. This can result in a CRI reduction in the overall emission. There is also some efficiency loss associated with the phosphor absorption and emission process, and repeating this process through re-absorption of the yellow/green light by the red phosphor results in additional efficiency losses.